If time is an issue you are best off with a motor driver module, like the one you identified from Solarbotics (the L298 one) or our own Rugged Motor Driver
. Really, if that will work depends upon the voltage you are applying and the current you want to pump into each phase. It sounds like 24V is the voltage given the power supply you bought but I'm not sure you mentioned what the per-phase coil current is for the motors you bought, nor what current you WANT to run them at to get the required torque.
Any more details on your motors or application?
The Gadget Shield
: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons